This invention relates to a double cylinder lock operable by a key from the interior or exterior, the key being retained in the interior cylinder when the lock bolt is extended.
Lock bolts on exterior doors are constructed with either a single lock cylinder or a dobule lock cylinder. A single lock cylinder requires a key to actuate it from the exterior, but allows manual actuation as by a thumb turn at the interior. A double lock cylinder requires a key to actuate it from the interior as well as the exterior. There are environments in which the single lock cylinder is preferable and other environments where the double lock cylinder is preferable. Usually this is determined by the needs of the owner of the premises and/or building code requirements.
Where double cylinder locks are employed, it is desirable and sometimes required that the key not be extractable from the interior cylinder when the lock bolt is extended. This is to assure ready unlocking capacity for rapid exit under panic conditions such as fire.
Achieving a double cylinder lock with interior key retention, which will not go "out of time" and cause inner key retention when unlocked has been a challenge to the trade.